The present invention relates generally to the field of fiber optic communication systems and more particularly to amplification of one or more multiplexed channels of different wavelengths carried simultaneously on a single fiber.
Optical amplifiers are widely used in fiber communication systems and enable the transmission of optical signals over thousands of kilometers. An optical amplifier typically includes a light amplifying medium, such as a fiber doped with a rare-earth ion that has an unpaired electron, for example, an Erbium doped fiber. Not many dopant ions are needed, just a few for every thousand atoms of silica in the fiber. An optical source, such as a particular type of diode laser commonly known as a pump, operating at a 980 or 1480 nanometer wavelength (nm), excites ions in the doped fiber raising them from a base or ground state to an excited state. In this excited or laser-pumped state, the ions are poised to radiate a photon and return to the base state whenever another photon of the same resonant wavelength impinges on it. When a photon of an incoming signal impinges on an excited ion within the doped fiber the one incoming photon becomes two, stimulating further emissions of light from the excited ions and starting a photon chain reaction within the doped fiber. Thus, the intensity of the incoming signal is progressively increased along the length of the doped fiber, amplifying the signal. The total output power of the amplifier, and thus the amplification or gain of the signal, depends primarily on the energy of light injected by the pump, which in turn depends on the power applied to the pump.
The introduction in recent years of wavelength division multiplexing (WDM), a scheme in which several optical signals or channels are transmitted simultaneously over a single path has led to the development of optical amplifiers, known as WDM amplifiers, that can operate over wide range of wavelengths. Ideally, the WDM amplifier provides a constant gain for each optical channel whatever the total input power or number of channels present in the input signal. However, this will not occur if the WDM amplifier is operated in a constant output power mode or in a constant pump current mode. Assuming the WDM amplifier is in saturation the total output power changes little with varying levels of input power. Also assuming that each input channel is at the same power level, as the number of channels is increased, the total available power must be shared between the additional channels reducing the gain on each channel. Thus, for a WDM amplifier to maintain a constant gain, the pump power must be adjusted for changes in either the number of channels or the total input power. Such changes generally require a technician to increase power to the pump, add pumps or replace the existing pump with a larger one.
In the first generation of fiber communication systems using WDM, the number of channels remained fixed for long periods. Thus, only infrequently was it necessary to reconfigure the system to add additional channels or to remove existing channels. Accordingly, there was ample time to adjust the pump power to the appropriate levels.
However, in the latest generation of applications, such as optical switching, it is frequently necessary to add or drop channels. This means input power to the WDM amplifier changes dynamically, and to maintain a constant gain for an output signal the pump power must also be adjusted dynamically. Moreover, in a system having a dynamically changing input power the transient response of the WDM amplifier becomes much more important. Transient response is a measure of the how quickly and how closely the output signal tracks changes in the input power. The key parameters for transient response are settling time and overshoot. Settling time is the time it takes for the output power to reach steady state in response to a change in the input power. Overshoot is the amount by which the signal goes above the steady state level.
Several approaches to providing WDM amplifiers having a constant gain and a fast transient response have been tried. However, none of these approaches have been wholly satisfactory. A key factor that complicates the ability to maintain accurate gain control and a fast transient response is differentiating the power of the output signal from the total power at the output. This is because the light amplifying medium typically generates spontaneous emissions or optical noise, which can be significant portion of the total output power when compared with the power of the output signal. In other words, total output power is equal to signal power plus the spontaneous emissions from the light amplifying medium.
One conventional design for WDM amplifiers uses a control or feedback loop in which two detectors, one at the input to the amplifier and one at the output of the amplifier, are used to control the power applied to the pump to maintain a constant gain. One draw back to this approach is that the output detector cannot distinguish between the spontaneous emission and the signal power leading to poor control of the signal gain. Another draw back is that when the input power is zero the pump power is also zero. This typically leads to overshoot when channels are added resulting in a poor transient response.
Accordingly, there is a need for an optical amplifier that is capable of providing substantially equal and constant gain to each channel of a signal having one or more multiplexed channels of different wavelengths. It is also desirable that the amplifier is capable of automatically adjusting to provide substantially equal and constant gain to each channel when additional channels are added or existing channels are removed. It is further desirable that the amplifier can provide a fast transient response to changes in input power, such as when channels are added or are removed.
The present invention provides a solution to these and other problems, and offers other advantages over the prior art.
The present invention relates to an apparatus and method for amplifying an incoming signal having one or more multiplexed channels of different wavelengths carried simultaneously on a single fiber that solves these problems.
According to one embodiment, an optical amplifier is provided. The optical amplifier includes an input coupler for receiving an incoming signal to the amplifier, a doped fiber coupled to the input coupler, a pump coupled to the doped fiber to provide an output to amplify the incoming signal within the doped fiber and a control system for controlling the pump. The incoming signal is split by the input coupler with, for example, about 99% of the power of the incoming signal being transmitted from the input coupler to the doped fiber and about 1% being transmitted to the control system. The control system has an optical detector for measuring power of the incoming signal, and a controller for adjusting electrical power applied to the pump based on the measured power of the incoming signal and predetermined criteria. The controller adjusts the electrical power applied to the pump as a linear function of the power of the incoming signal by including within the predetermined criteria a multiplier by which the measured power of the incoming signal is multiplied. The predetermined criteria also include an offset that is added to the product of the multiplier and the signal power to ensure that the electrical power applied to the pump, and therefor the power of the output of the pump, is non-zero even for zero signal power. Preferably, the incoming signal is made up of a number of multiplexed channels having different wavelengths and the controller is capable of automatically adjusting electrical power to the pump to provide equal and constant gain to each channel when additional channels are added or existing channels are removed. More preferably, the controller can provide a transient response of less than 200 xcexcs when channels are added or are removed.
In one version of this embodiment, the control system further includes a monitoring circuit for measuring the power of the output of the pump. This monitoring circuit can include either a second detector coupled to a coupler at the output of the pump or a facet monitor inside the pump. In this version, the electrical power applied to pump is adjusted so that the power of the output of the pump is equal to the sum of the product of the multiplier and the signal power and the offset.
In another aspect, the present invention is directed to a method of operating an optical amplifier to provide equal and constant gain to a signal made up of several multiplexed channels having different wavelengths. In the method, the amplifier receives the signal and measures the power of the signal. A controller adjusts electrical power applied to a pump based on the measured power of the signal and predetermined criteria. The signal is then combined with an output from the pump, and the combined signal and pump output passed through a doped fiber to amplify the signal. Preferably, the step of adjusting electrical power applied to the pump involves automatically adjusting power to provide equal and constant gain to each channel even when additional channels are added or existing channels are removed. More preferably, the electrical power applied to the pump is adjusted sufficiently fast to provide a transient response of less than 200 xcexcs when channels are added or are removed.
In one version, the step of adjusting the electrical power to the pump involves adjusting the electrical power applied to the pump as a linear function of the power of the incoming signal. Generally, the predetermined criteria include a multiplier, and the step of adjusting the electrical power to the pump involves multiplying the power of the incoming signal by the multiplier. The predetermined criteria can further include an offset that is added to the product of the multiplier and the signal power so that the electrical power applied to the pump, and therefor the power of the output of the pump, is non-zero even for zero signal power.
In yet another aspect, the present invention is directed to an optical amplifier for use in a fiber communication system that can provide equal and constant gain to each channel in a signal having several channels. The amplifier includes a light amplifying medium connected to an input coupler to amplify the signal, an excitation light source coupled to the light amplifying medium to amplify a signal within the light amplifying medium, and control means for controlling the excitation light source. Generally, the control means includes means for measuring power of the incoming signal, and means for adjusting electrical power to the excitation light source based on the measured power of the signal and predetermined criteria. Preferably, the control means can automatically adjust power to the excitation light source to provide equal and constant gain to each channel even when additional channels are added or existing channels are removed. More preferably, the control means can provide a transient response of less than 200 xcexcs when channels are added or are removed.